Adapting a number of aggregation levels for contorl channel elements

ABSTRACT

Methods and apparatuses are provided for a User Equipment (UE) to receive control information. A Downlink Control Information (DCI) format to be monitored is identified. A number of Enhanced Physical Downlink Control Channel (EPDCCH) candidates are identified based on the identified DCI format. Control information is received based on the identified number of EPDCCH candidates.

PRIORITY

This application is a Continuation application of U.S. patentapplication Ser. No. 14/017,934, filed on Sep. 4, 2013, which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Application No.61/696,501, which was filed in the United States Patent and TrademarkOffice on Sep. 4, 2012, the contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wireless communicationsystems and, more specifically, to the transmission and reception ofPhysical DownLink Control CHannels (PDCCHs).

2. Description of the Art

A conventional communication system includes a DL that conveystransmission signals from transmission points such as Base Stations (BSor NodeBs) to User Equipments (UEs) and an UpLink (UL) that conveystransmission signals from UEs to reception points such as NodeBs. A UE,also known as a terminal or a mobile station, is fixed or mobile and isa cellular phone or a Personal Computer device, for example. A NodeB,also known as an access point, is generally a fixed station.

DL signals include data signals carrying information content, controlsignals carrying DL Control Information (DCI), and Reference Signals(RSs), which are also known as pilot signals. A NodeB transmits datainformation or DCI to UEs through respective Physical DL Shared CHannels(PDSCHs) or PDCCHs.

UL signals also include data signals, control signals and RSs. A UEtransmits data information or UL Control Information (UCI) to a NodeBthrough a respective Physical Uplink Shared CHannel (PUSCH) or aPhysical Uplink Control CHannel (PUCCH).

A PDSCH transmission to a UE or a PUSCH transmission from a UE is inresponse to dynamic scheduling or to Semi-Persistent Scheduling (SPS).With dynamic scheduling, a NodeB conveys to a UE a DCI format through arespective PDCCH. With SPS, a PDSCH or a PUSCH transmission isconfigured to a UE by a NodeB through higher layer signaling, such asRadio Resource Control (RRC) signaling, in which case the schedulingoccurs at time instances and with parameters instructed by the higherlayer signaling.

A NodeB may also transmit multiple types of RSs including a UE-Common RS(CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS(DMRS). A CRS is transmitted over substantially an entire DL systemBandWidth (BW) and can be used by all UEs to demodulate data, controlsignals, or perform measurements. To reduce an overhead associated witha CRS, a NodeB may transmit a CSI-RS with a smaller density in a timeand/or frequency domain than a CRS for UEs to perform measurements, andtransmit a DMRS only in a BW of a respective PDSCH. A UE can use a DMRSto demodulate information in a PDSCH.

FIG. 1 illustrates a conventional transmission structure for a DLTransmission Time Interval (TTI).

Referring to FIG. 1, a DL TTI includes one subframe 110 which includestwo slots 120 and a total of N_(symb) ^(DL) symbols for transmittingdata information, DCI, or RS. First M_(symb) ^(DL) subframe symbols areused to transmit PDCCHs and other control channels (not shown) 130.Remaining N_(symb) ^(DL)-M_(symb) ^(DL) subframe symbols are primarilyused to transmit PDSCHs 140. A transmission BW includes frequencyresource units referred to as Resource Blocks (RBs). Each RB includesN_(sc) ^(RB) sub-carriers, or Resource Elements (REs), and a UE isallocated M_(PDSCH) RBs for a total of M_(sc) ^(PDSCH)=M_(PDSCH)·N_(sc)^(RB) REs for a PDSCH transmission BW. An allocation of one RB in afrequency domain and of one slot and two slots (one subframe) in a timedomain will be referred to as a Physical RB (PRB) and a PRB pair,respectively. Some REs in some symbols contain CRS 150, CSI-RS or DMRS.

DCI can serve several purposes. A DCI format in a respective PDCCH mayschedule a PDSCH or a PUSCH providing data or control information to orfrom a UE, respectively. Another DCI format in a respective PDCCH mayschedule a PDSCH providing System Information (SI) to a group of UEs fornetwork configuration parameters, a response to a Random Access (RA) byUEs, or paging information, for example. Another DCI format may provideTransmission Power Control (TPC) commands to a group of UEs for SPStransmissions in respective PUSCHs or PUCCHs.

A DCI format includes Cyclic Redundancy Check (CRC) bits in order for aUE to confirm a correct detection. The DCI format type is identified bya Radio Network Temporary Identifier (RNTI) that scrambles the CRC bits.The RNTI is a Cell RNTI (C-RNTI) for a DCI format scheduling a PDSCH ora PUSCH to a single UE. The RNTI is an SI-RNTI for a DCI formatscheduling a PDSCH conveying SI to a group of UEs. The RNTI is anRA-RNTI for a DCI format scheduling a PDSCH providing a response to a RAfrom a group of UEs. The RNTI is a P-RNTI for a DCI format scheduling aPDSCH paging a group of UEs. The RNTI is a TPC-RNTI for a DCI formatproviding TPC commands to a group of UEs. Each RNTI type is configuredto a UE through higher layer signaling from a NodeB (and the C-RNTI isunique for each UE).

FIG. 2 illustrates a conventional encoding and transmission process fora DCI format at a NodeB transmitter.

Referring to FIG. 2, an RNTI of a DCI format masks a CRC of a codewordin order to enable a UE to identify a DCI format type. A CRC 220 of(non-coded) DCI format bits 210 is computed and is subsequently masked230 using an eXclusive OR (XOR) operation between CRC and RNTI bits 240,which is XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. A masked CRC isthen appended to DCI format bits 250, channel coding is performed 260such as by using a convolutional code, rate matching 270 is performed toallocated resources, and modulation 280 and transmission of a controlsignal 290 are performed by interleaving. For example, both a CRC and anRNTI include 16 bits.

FIG. 3 illustrates a conventional reception and decoding process for aDCI format at a UE receiver.

Referring to FIG. 3, a received control signal in step 310 isdemodulated and resulting bits are de-interleaved in step 320, a ratematching applied at a NodeB transmitter is restored in step 330, andcontrol information is subsequently decoded 340. DCI format bits in step360 are then obtained after extracting CRC bits in step 350 which arethen de-masked in step 370 through an XOR operation with an RNTI in step380. A UE then performs a CRC test in step 390. If a CRC test passes, aUE considers the DCI format to be valid and determines parameters forPDSCH reception or PUSCH transmission. If a CRC test does not pass, a UEdisregards a presumed DCI format.

A NodeB separately codes and transmits a DCI format in a respectivePDCCH. To avoid a first PDCCH transmission blocking a second PDCCHtransmission, a location of each PDCCH in a DL control region is notunique. Consequently, a UE needs to perform multiple decoding operationsper subframe to determine whether there is a PDCCH intended for the UE.REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) ina logical domain. CCE aggregation levels may include, for example, 1, 2,4, and 8 CCEs.

FIG. 4 illustrates a conventional transmission process of DCI formats inrespective PDCCHs.

Referring to FIG. 4, encoded DCI format bits are mapped to PDCCH CCEs ina logical domain. A first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403,and CCE4 404 are used to transmit a PDCCH to UE1. The following 2 CCEs(L=2), CCE5 411 and CCE6 412, are used to transmit a PDCCH to UE2. Thefollowing 2 CCEs (L=2), CCE7 421 and CCE8 422, are used to transmit aPDCCH to UE3. A last CCE (L=1), CCE9 431, is used to transmit a PDCCH toUE4. DCI format bits are scrambled in step 440 by a binary scramblingcode and are subsequently modulated in step 450. Each CCE is furtherdivided into Resource Element Groups (REGs). For example, a CCEconsisting of 36 REs is divided into 9 REGs, each consisting of 4 REs.Interleaving in step 460, for example block interleaving, is appliedamong REGs which, assuming Quadrature Phase Shift Keying (QPSK)modulation for a PDCCH, include blocks of 4 QPSK symbols. A resultingseries of QPSK symbols is shifted by J symbols in step 470, and eachQPSK symbol is mapped to an RE in step 480 in a control region of a DLsubframe. Therefore, in addition to the CRS, 491 and 492, and othercontrol channels (not shown), REs in a DL control region contain QPSKsymbols corresponding to DCI format for UE1 494, UE2 495, UE3 496, andUE4 497.

For a PDCCH decoding process, a UE may determine a search space forcandidate PDCCH transmissions after it restores CCEs in a logical domainaccording to a UE-common set of CCEs (Common Search Space or CSS) andaccording to a UE-dedicated set of CCEs (UE-Dedicated Search Space orUE-DSS). A CSS may include first C CCEs in a logical domain, which isused to transmit PDCCHs for DCI formats associated with UE-commoncontrol information and use a SI-RNTI, a P-RNTI, a TPC-RNTI, to scramblerespective CRCs. A UE-DSS includes remaining CCEs, which are used totransmit PDCCHs for DCI formats associated with UE-specific controlinformation and use respective C-RNTIs to scramble respective CRCs.

CCEs of a UE-DSS is determined according to a pseudo-random functionhaving as inputs UE-common parameters, such as a subframe number or atotal number of CCEs in a subframe, and UE-specific parameters such as aC-RNTI. For example, for CCE aggregation level of L ε{1,2,4,8} CCEs, theCCEs corresponding to PDCCH candidate m are given by

CCEs for PDCCH candidate m=L·{(Y _(k) +m) mod └N _(CCE,k)/L┘}+i  Equation 1

where N_(CCE,k) is a total number of CCEs in subframe k, i=0, . . .,L−1, m=0, . . . M_(C) ^((L))−1, and M_(C) ^((L)) is a number of PDCCHcandidates to monitor in a UE-DSS. Values of M_(C) ^((L)) forLε{1,2,4,8} are, respectively, {6, 6, 2, 2}, for example. For a UE-DSS,Y_(k)=(A·Y_(k-1))mod D where Y⁻¹=C−RNT1≠0, A=39827 and D=65537. For aCSS, Y_(k)=0.

A conventional DL control region may occupy a maximum of M_(symb)^(DL)=3 subframe symbols and a PDCCH is transmitted substantially overan entire DL BW. Consequently, network functionalities such as extendedPDCCH capacity in a subframe and PDCCH interference coordination in afrequency domain, which are needed in several cases, cannot besupported. One such case is a use of Remote Radio Heads (RRHs) in anetwork where a UE may receive DL signals either from a macro-NodeB orfrom an RRH. If RRHs and a macro-NodeB share a same cell identity,cell-splitting gains do not exist and expanded PDCCH capacity is neededto accommodate PDCCH transmissions from both a macro-NodeB and RRHs.Another case is for heterogeneous networks where DL signals from apico-NodeB experience strong interference from DL signals from amacro-NodeB, and interference coordination in a frequency domain amongNodeBs is needed.

A direct extension of a conventional DL control region size to more thanM_(symb) ^(DL)=3 subframe symbols is not possible at least due to a needfor support of conventional UEs, which cannot be aware or support suchextension. An alternative is to support DL control signaling in a PDSCHregion by using individual PRB pairs. A PDCCH transmitted in one or morePRB pairs of a conventional PDSCH region will be referred to as EnhancedPDCCH (EPDCCH).

FIG. 5 illustrates a conventional EPDCCH transmission structure in a DLsubframe.

Referring to FIG. 5, although EPDCCH transmissions start immediatelyafter PDCCH transmissions 510 and are over all remaining subframesymbols, they may instead always start at a fixed location, such as thefourth subframe symbol, and may extend over a part of the remainingsubframe symbols. EPDCCH transmissions occur in four PRB pairs, 520,530, 540, and 550 while the remaining PRB pairs are used for PDSCHtransmissions 560, 562, 564, 566, 568.

A UE can be configured by higher layer signaling from a NodeB for one ormore sets of PRB pairs that may convey EPDCCHs. The transmission of anEPDCCH to a UE is in one or a few PRB pairs, if a NodeB has accurate CSIfor the UE and can perform Frequency Domain Scheduling (FDS) orbeam-forming. This transmission is in many PRB pairs, possibly alsousing transmitter antenna diversity, if accurate CSI per PRB pair is notavailable at the NodeB. An EPDCCH transmission over one or a few PRBpairs will be referred to as localized or non-interleaved, while anEPDCCH transmission over many PRB pairs will be referred to asdistributed or interleaved.

An exact EPDCCH search space design is not material to the presentinvention and may follow same principles as a PDCCH search space design.An EPDCCH includes respective CCEs, referred to as ECCEs, and a numberof EPDCCH candidates exist for each possible ECCE aggregation levelL_(E). For example, L_(E)ε{1,2,4,8} ECCEs exist for localized EPDCCHsand L_(E)ε{1,2,4,8,16} ECCEs exist for distributed EPDCCHs. An ECCE mayor may not have the same size as a conventional CCE.

A number of EPDCCH REs per PRB pair varies depending on a size of aconventional DL control region, defined by a number of M_(symb) ^(DL)subframe symbols in FIG. 1, on a number of CSI-RS REs and CRS REs, forexample. This variation can be addressed either by maintaining a sameECCE size and possibly having a variable number of ECCEs per PRB pair indifferent subframes (and possibly also having some REs that cannot beallocated to an ECCE) or by maintaining a same number of ECCEs per PRBpair and having a variable ECCE size.

An ECCE size is defined by a respective number of REs available fortransmitting an EPDCCH (excluding REs used to transmit other signals ina PRB pair), and is different than a fixed, maximum ECCE size. A maximumECCE size is obtained by assuming that no signals, other than DMRSassociated with demodulation of EPDCCHs, are transmitted in PRB pairsused to transmit EPDCCHs. Then, for a PRB pair including N_(sc) ^(RB)=12REs, N_(symb) ^(DL)=14 subframe symbols, and 24 REs for DMRStransmission, there are N_(sc) ^(RB)·N_(symb) ^(DL)−24=144 REs availablefor transmitting EPDCCHs, and the maximum ECCE size is 36 REs for 4ECCEs per PRB pair.

FIG. 6 illustrates conventional variations in an ECCE size per subframeassuming four ECCEs per PRB pair.

Referring to FIG. 6, in a first realization for a number of REs totransmit EPDCCHs 610, a conventional DL control region spans a firstthree subframe symbols 620 and there is a first number of DMRS REs 630,CSI-RS REs 632, and CRS REs 634. An ECCE size is 21 REs for 4 ECCEs perPRB pair. In a second realization for a number of REs to transmitEPDCCHs 640, a conventional DL control region spans a first one subframesymbol 650, and there is a second number of DMRS REs 660 and CRS REs 662(no CSI-RS REs). An ECCE size is 30 REs for 4 ECCEs per PRB pair, orabout 43% more than in the first realization. Larger variations in anECCE size may also exist.

As an ECCE size may vary per subframe, and a minimum ECCE aggregationlevel required to reliably detect an EPDCCH may also vary. A thresholdfor an ECCE size T_(RE) is defined, and ECCE aggregation levels aretwice the aggregation levels for when an ECCE size is greater than orequal to T_(RE), when an ECCE size is less than T_(RE). For example, ifL_(E) ε{1,2,4,8} are ECCE aggregation levels for transmitting adistributed EPDCCH with an ECCE size greater than or equal to T_(RE),then L_(E) ε{2,4,8,16} are ECCE aggregation levels for transmitting adistributed EPDCCH with an ECCE size less than T_(RE).

Using a single T_(RE) value fails to properly address a need for a UE todetect different EPDCCHs conveying DCI formats with differentinformation payloads. For example, a first DCI format scheduling a PUSCHmay have a payload of 43 bits while a second DCI format scheduling aPDSCH may have a payload of 58 bits. Then, for QPSK and for T_(RE)=26,even though an aggregation level of one ECCE may sufficiently convey thefirst DCI format, as a respective maximum code rate is 0.83, anaggregation level of one ECCE cannot sufficiently convey the second DCIformat as a respective maximum code rate is 1.12 and T_(RE)=35 would berequired for a maximum code rate of about 0.83 for the second DCIformat.

Using a single T_(RE) value also fails to account for a variablemodulation scheme to transmit an EPDCCH. The modulation scheme is one ofthe components determining a respective code rate for transmitting aninformation payload of a DCI format in an EPDCCH for an ECCE aggregationlevel.

Using a single T_(RE) value further fails to account for variations inan information payload of a DCI format according to a presence orabsence of configurable information fields, variations of resourceallocation, or other information fields according to a DL or ULoperating bandwidth. A UE can determine a DL or UL operating bandwidthby receiving system information transmitted by a NodeB.

Therefore, a need exists in the art to define multiple thresholds ofECCE sizes, each threshold corresponding to one or more DCI formats a UEattempts to detect, for adjusting respective ECCE aggregation levels forEPDCCH candidates depending on whether an ECCE size is less than athreshold or greater than or equal to a threshold.

A need exists in the art to define a threshold of an ECCE size foradjusting respective ECCE aggregation levels for EPDCCH candidatesdepending on a respective modulation scheme.

A need exists in the art to adjust ECCE aggregation levels for EPDCCHcandidates for different DCI formats depending on a maximum bandwidththat can be scheduled, and on the DCI format.

A need exists in the art to adjust ECCE aggregation levels for EPDCCHcandidates for different DCI formats depending on a code rate for arespective DCI format.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been designed to solve at leastthe aforementioned limitations and problems in the prior art, andprovides methods and an apparatus for a base station to transmit and fora UE to decode a PDCCH.

In accordance with an aspect of the present invention, a method isprovided for a UE to receive control information. A DCI format to bemonitored is identified. A number of EPDCCH candidates are identifiedbased on the identified DCI format. Control information is receivedbased on the identified number of EPDCCH candidates.

In accordance with another aspect of the present invention, a method isprovided for a base station to transmit control information. Controlinformation is transmitted. The control information is received by a UEbased on a number of EPDCCH candidates. The number of EPDCCH candidatesis identified by the UE based on a DCI format to be monitored.

In accordance with another aspect of the present invention, a UE isprovided to receive control information. The UE includes a controllerconfigured to identify a DCI format to be monitored, and identify anumber of EPDCCH candidates based on the identified DCI format. The UEalso includes a transceiver configured to receive control informationbased on the identified number of EPDCCH candidates.

In accordance with another aspect of the present invention, a basestation is provided to transmit control information. The base stationincludes a transceiver configured to transmit control information. Thecontrol information is received by a UE based on a number of EPDCCHcandidates. The number of EPDCCH candidates is identified by the UEbased on a DCI format to be monitored.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a structure for a DL TTI;

FIG. 2 illustrates an encoding process for a DCI format;

FIG. 3 illustrates a decoding process for a DCI format;

FIG. 4 illustrates a transmission process of DCI formats in respectivePDCCHs.

FIG. 5 illustrates an EPDCCH transmission structure in a DL TTI;

FIG. 6 illustrates variations in an ECCE size per subframe assuming fourECCEs per PRB pair;

FIG. 7 illustrates a use of multiple thresholds for an ECCE size, whereeach threshold is associated with one or more DCI formats a UE attemptsto detect in a subframe, for adjusting a number of ECCE aggregationlevels, according to an embodiment of the present invention;

FIG. 8A illustrates a use of a nominal set of ECCE aggregation levels ora use of a supplemental set of ECCE aggregation levels depending on aDCI format among multiple DCI formats, according to an embodiment of thepresent invention;

FIG. 8B illustrates a configuration of an ECCE threshold value persubframe and a configuration of a minimum set of PRB pairs fortransmitting EPDCCHs per subframe, according to an embodiment of thepresent invention;

FIG. 9 illustrates a dependence of an ECCE aggregation level on anEPDCCH modulation scheme, according to an embodiment of the presentinvention;

FIG. 10 illustrates a dependence of an ECCE aggregation level for whicha UE attempts to detect an EPDCCH conveying a DCI format on an operatingbandwidth, according to an embodiment of the present invention;

FIG. 11 illustrates a dependence of EPDCCH candidates per ECCEaggregation level on a DCI format conveyed by an EPDCCH a UE attempts todetect, according to an embodiment of the present invention;

FIG. 12 illustrates an encoding and transmission process for a DCIformat at a NodeB transmitter incorporating an adaptation of ECCEaggregation levels per subframe, according to an embodiment of thepresent invention; and

FIG. 13 illustrates a reception and decoding process for a DCI format ata UE receiver incorporating an adaptation of ECCE aggregation levels persubframe, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. This present invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will fully convey thescope of the present invention to those skilled in the art. Detaileddescriptions of well-known functions and structures incorporated hereinis omitted for the sake of clarity and conciseness.

Although embodiments of the present invention will be described withreference to Orthogonal Frequency Division Multiplexing (OFDM), theyalso are generally applicable to all Frequency Division Multiplexing(FDM) transmissions, and particularly applicable to Discrete FourierTransform (DFT)-spread OFDM.

The present invention considers methods and an apparatus for adjusting anumber of ECCE aggregation levels in a subframe according to an ECCEsize, a modulation scheme for an EPDCCH conveying a DCI format, or anoperating BW and a DCI format size, and for assigning a different numberof EPDCCH candidates per ECCE aggregation level according to a DCIformat size. As described in FIG. 6, an ECCE size refers to a number ofREs available for transmitting EPDCCHs in a PRB pair assuming a fixednumber of ECCEs per PRB pair, such as 4 ECCEs per PRB pair. Therefore,defining multiple thresholds for ECCE sizes is also equivalent todefining multiple thresholds for the number of REs available fortransmitting EPDCCHs in a PRB pair (for 4 ECCEs per PRB pair, the latteris a multiple of 4 of the former).

A first embodiment of the present invention considers a definition ofmultiple thresholds of ECCE sizes, each threshold corresponding to oneor more DCI formats a UE attempts to detect, for adjusting respectiveECCE aggregation levels in a subframe depending on whether an ECCE sizeis less than a threshold or greater than or equal to a threshold.

The first embodiment will now be described with respect to a DCI Format0 scheduling a PUSCH transmission from a UE without spatialmultiplexing, a DCI Format 4 scheduling a PUSCH transmission from a UEwith or without spatial multiplexing, a DCI Format 1A scheduling a PDSCHtransmission to a UE without spatial multiplexing, and a DCI Format 2Cscheduling a PDSCH transmission to a UE with or without spatialmultiplexing. DCI Format 0 and DCI Format 1A are assumed to be designedso that they have the same size, and they are jointly referred to as DCIFormat 0/1A. Due to support of scheduling PUSCH or PDSCH transmissionswith spatial multiplexing, DCI Format 4 and DCI Format 2C, respectively,have larger sizes than DCI Format 0/1A. Indicative information payloadsfor DCI Format 0/1A, DCI Format 4 and DCI Format 2C are 43 bits, 51bits, and 58 bits, respectively, for a large operating BW of 50 RBs, or37 bits, 45 bits, and 46 bits, respectively, for a large operating BW of6 RBs.

Assuming QPSK modulation (conveying two information bits per RE) for anEPDCCH transmission and a target maximum coding rate less than R_(max)to ensure a detection reliability of a respective DCI format, and for aDCI format information payload of O_(DCI) bits, a minimum ECCE size isobtained as S_(min)−O_(DCI)/(2·R_(max)). Denoting by S_(min) ^(0/1A) aminimum ECCE size for DCI Format 0/1A and by S_(min) ^(2C) a minimumECCE size for DCI Format 2C, a relative difference between the twominimum ECCE sizes is (S_(min) ^(2c)−S_(min) ^(0/1A))/S_(min) ^(0/1A),or about 35% for respective payloads of 43 bits and 58 bits.

Considering that typical variations in an ECCE size due to a presence orabsence of other signals are less than 35%, it becomes apparent thatusing a single T_(RE) value to define ECCE aggregation levels in asubframe for reliable detection for both DCI Format 0/1A and DCI Format2C will result in either an overestimation of a minimum ECCE aggregationlevel for the former, or an underestimation of a minimum ECCEaggregation levels for the latter. For example, if an ECCE sizethreshold of T_(RE)=26 REs is used (or, equivalently, a threshold of4×26=104 available REs for transmitting EPDCCHs in a PRB pair is used),for an EPDCCH transmission with one ECCE, a channel coding rate of 0.83applies to a DCI Format 0/1A while a channel coding rate of 1.12 appliesto a DCI Format 2C. Therefore, reliable detection for DCI Format 0/1A ispossible for an EPDCCH transmission with an aggregation level of oneECCE (at least for a UE experiencing a relatively high DL SINR) whilereliable detection for DCI Format 2C is not possible for a largeoperating BW of 50 RBs. For scheduling in a small operating BW of 6 RBs,as the size of DCI Format 2C (46 bits) is similar to that of DCI Format1A for scheduling in a large operating BW of 50 RBs (43 bits), reliabledetection for DCI Format 2C is possible with an EPDCCH transmissionhaving an aggregation level of one ECCE. This also applies for DCIFormat 1A as its size (37 bits) is less than the size for scheduling ina large operating BW of 50 RBs (43 bits).

Reliable detection for both DCI Format 0/1A and DCI Format 2C isachieved if T_(RE) was increased, for example to a value of 36. However,this may substantially reduce a use for an aggregation level of one ECCEas, for four ECCEs per PRB pair, a number of available REs per PRB pairis rarely equal to 144 (for example, due to presence of other signalssuch as CRS or CSI-RS or conventional control signals). An increaseT_(RE) value may also often require unnecessary coding redundancies (lowchannel coding rates) for a transmission of DCI formats with relativelylow information payloads, such as DCI Format 0/1A. Nevertheless, anadditional threshold for an ECCE size can be introduced to accommodateone or more of second DCI formats a UE is configured to detect.

An immediate implication of the above inability to efficiently supportboth DCI Format 0/1A and DCI Format 2C with an aggregation level of oneECCE, is that different DCI formats should be allocated a differentnumber of ECCE aggregation levels in a subframe. For example, aspreviously described, for DCI Format 0/1A an aggregation level of oneECCE is supported for all operating BWs while for DCI Format 2C anaggregation level of one ECCE is not supported for large operating BWs.

FIG. 7 illustrates a use of multiple thresholds for an ECCE size, whereeach threshold is associated with one or more DCI formats a UE attemptsto detect in a subframe, for adjusting a number of ECCE aggregationlevels, according to an embodiment of the present invention.

Referring to FIG. 7, at least a first ECCE size threshold associatedwith a first set of one or more DCI formats, such as DCI Format 0/1A,and a second ECCE size threshold T_(RE) ², associated with a second setof one or more DCI formats, such as DCI Format 2C, are used for ECCEaggregation levels in a subframe in step 710. The values of T_(RE) ¹ andT_(RE) ² is specified in an operation of a communication system or isconfigured to a UE from a NodeB by higher layer signaling. Assuming thatT_(RE) ²>T_(RE) ¹, if an ECCE size in a subframe is less than T_(RE) ¹in step 720, an aggregation level of one ECCE may not be used totransmit an EPDCCH for any DCI format in step 730. If additionally anECCE size is less than T_(RE) ²/2, an aggregation level of two ECCEs maynot be used for an EPDCCH transmission conveying a DCI format from thesecond set. Nevertheless, for simplicity, it is assumed that T_(RE)²<2·T_(RE) ¹ and that a smallest ECCE size is greater than T_(RE) ²/2(and hence greater than T_(RE) ²/4). If an ECCE size in a subframe isgreater than or equal to T_(RE) ¹ but less than T_(RE) ² in step 740, anaggregation level of one ECCE is used for an EPDCCH transmission toconvey a DCI format from the first set but may not be used for an EPDCCHtransmission to convey a DCI format from the second set in step 750. Ifan ECCE size in a subframe is greater than or equal to T_(RE) ², anaggregation level of one ECCE is used for an EPDCCH transmission toconvey a DCI format either from the first set of DCI formats or from thesecond set of DCI formats in step 760.

When an ECCE size is less than T_(RE) ¹ or T_(RE) ², an inability to usean aggregation level of one ECCE to transmit an EPDCCH conveying arespective DCI format additionally requires that a maximum ECCEaggregation level is doubled. For example, if a nominal set of ECCEaggregation levels is defined as L_(E) ε{1,2,4,8} ECCEs then, when anaggregation level of one ECCE is not supported for a DCI format in asubframe, a supplemental set of ECCE aggregation levels is defined asL_(E) ε{2,4,8,16}. Whether a UE considers a nominal set of ECCEaggregation levels which includes an aggregation level of one ECCE andhas a first maximum aggregation level or considers a supplemental set ofECCE aggregation levels which does not include an aggregation level ofone ECCE and has a second maximum aggregation level, wherein the secondmaximum is twice the first maximum, depends on whether an ECCE size in arespective subframe is smaller or greater than or equal to a thresholdcorresponding to a respective DCI format.

FIG. 8A illustrates a use of a nominal set of ECCE aggregation levels ora use of a supplemental set of ECCE aggregation levels depending on aDCI format among multiple DCI formats, according to an embodiment of thepresent invention.

Referring to FIG. 8A, at least a first ECCE size threshold T_(RE) ¹,associated with a first set of one or more DCI formats, such as DCIFormat 0/1A, and a second ECCE size threshold T_(RE) ², associated witha second set of one or more DCI formats, such as DCI Format 2C, arere-used for ECCE aggregation levels in a subframe in step 810. If anECCE size in a subframe is less than T_(RE) ¹ in step 820, asupplemental set of ECCE aggregation levels is used for a transmissionof both a DCI format from a first set of DCI formats and of a DCI formatfrom a second set of DCI formats in respective EPDCCHs in step 830. Ifan ECCE size in a subframe is greater than or equal to T_(RE) ¹ but lessthan T_(RE) ² in step 840, a nominal set of ECCE aggregation levels isused for a transmission of a DCI format from a first set of DCI formatsand a supplemental set of ECCE aggregation levels is used for atransmission of a DCI format from a second set of DCI formats inrespective EPDCCHs in step 850. If an ECCE size in a subframe is greaterthan or equal to T_(RE) ², a nominal set of ECCE aggregation levels isused for a transmission of both a DCI format from a first set of DCIformats and of a DCI format from a second set of DCI formats inrespective EPDCCHs in step 860.

A UE may only decode EPDCCH candidates for ECCE aggregation levels usedfor an EPDCCH transmission. Additionally, ECCE aggregation levels whenone ECCE can be used for an EPDCCH transmission conveying a DCI formatare L_(E) ε{1,2,4,8} ECCEs. When one CCE cannot be used for an EPDCCHtransmission conveying a same DCI format, ECCE aggregation levels areL_(E) Σ{2,4,8,16} ECCEs.

An ECCE size threshold, or equivalently a threshold for a number of REsin a PRB pair for EPDCCH transmissions, may also depend on a subframe. Areason for such dependence is that a UE may not be aware of all REs in aPRB pair that may not be available for transmitting EPDCCHs. Forexample, a configuration of CSI-RS in a subframe is UE-specific and a UEmay not be aware of REs associated with CSI-RS in a subframe to otherUEs. Then, as a NodeB may not transmit EPDCCH in REs associated withCSI-RS, a UE that is unaware of REs associated with CSI-RS will treatsuch REs as conveying EPDCCH thereby assuming an ECCE size greater thanan actual one (in addition to experiencing a degradation in a detectionreliability of an EPDCCH as REs associated with CSI-RS are treated asconveying EPDCCH).

To circumvent the above problem, a NodeB may either configure by higherlayer signaling all CSI-RS instances to a UE, while differentiatingamong the ones intended and the ones not intended to a UE, or configureper subframe, in a set of subframes, an ECCE size threshold. The set ofsubframes includes a number of subframes, such as 10 subframes, and canrepeat every such number of subframes until re-configured. In the lattercase, a NodeB may configure a larger ECCE size threshold in subframeswith transmissions of signals a UE is unaware of than in subframes withtransmissions of signals that are fully known to a UE. For example, twoECCE sizes can be defined and a NodeB can indicate (for example, with abinary ‘0) the subframes for which the first ECCE size applies and canindicate (for example, with a binary ‘1’) the subframes for which thesecond ECCE size applied. A NodeB may also configure a UE with differentEPDCCH candidates per ECCE aggregation level per subframe so that insubframes with REs associated with signals a UE is unaware of, moreEPDCCH candidates are allocated to higher ECCE aggregation levels.

In addition to an ECCE size threshold depending on a subframe, a set ofPRB pairs used to transmit EPDCCH may also depend on a subframe in orderto account for variations in a number of REs available for transmittingEPDCCHs per subframe. In principle, to accommodate a same minimumrequirement in REs for transmitting EPDCCHs in a subframe, if an ECCEsize (or a number of REs per PRB pair available for transmitting EPDCCHsper subframe to a UE) decreases by a factor of X, a respective number ofPRB pairs should increase by a factor of 1/X. Therefore, if a firstminimum number of PRB pairs is used in a first subframe for transmittingEPDCCHs when an ECCE size has a first value, a second minimum number ofPRB pairs is used in a second subframe for transmitting EPDCCHs when anECCE size has a second value. For example, a minimum of 4 PRB pairs isused to transmit EPDCCHs in a subframe having an ECCE size greater thanor equal to 26 REs and a minimum of 8 PRB pairs is used to transmitEPDCCHs in a subframe having an ECCE size less than 26 REs. The minimumnumber of PRB pairs used to transmit EPDCCHs in a subframe is implicitlyderived from a number of REs per PRB pair available for transmittingEPDCCHs per subframe or from an ECCE size (or from an ECCE sizethreshold), with a reference value being configured or specified in asystem operation for a DL BW, or it is separately configured to a UE persubframe.

FIG. 8B illustrates a configuration of an ECCE threshold value persubframe and a configuration of a minimum set of PRB pairs fortransmitting EPDCCHs per subframe, according to an embodiment of thepresent invention.

Referring to FIG. 8B, a PRB pair used to transmit EPDCCHs contains DMRSREs 8A10, CRS REs 8A12, and REs for CCHs 8A16. A UE is assumed to befully aware of all such REs. The PRB pair also contains CSI-RS REs 8A14that the UE is not aware of (in general, a UE may not be aware of afirst number of CSI-RS REs and be aware of a second number of CSI-RS REsin a PRB pair). As a consequence of not being aware of all CSI-RS REs, aUE may assume an ECCE size of 30 REs when an actual ECCE size (for UEsfully aware of CSI-RS REs) is 26 REs. To avoid this problem, instead ofa UE computing on its own an ECCE threshold value, it is configured by aNodeB an ECCE threshold value which in the example of FIG. 8B may informof the actual ECCE size or even of a less than the actual ECCE size inorder to account for a degradation in an EPDCCH detection reliability.To account for a variable ECCE size per subframe, whether it is onecomputed by a UE or one configured to a UE by higher layer signalingfrom a NodeB, a minimum number of PRB pairs used to transmit EPDCCHs mayalso vary per subframe. For example, for a DL BW of 50 PRB pairs, insubframe k₁, a minimum of two PRB pairs (PRB pair 3 and PRB pair 33) areused to transmit EPDCCHs while in subframe k₂ (with an ECCE size lessthan in subframe k₁), a minimum of four PRB pairs (PRB pair 3, PRB pair18, PRB pair 33, and PRB pair 48) are used to transmit EPDCCHs. A samecondition occurs when an EPDCCH transmission is over a small number oftransmission symbols per subframe as a number of respective REs per PRBpair is also small.

The second embodiment of the present invention considers a dependence ofECCE aggregation levels to an order Q_(m) of a modulation scheme usedfor an EPDCCH transmission.

A modulation order Q_(m) is a measure of a spectral efficiency. QPSKconveys two information bits per RE (Q_(m)=2) while Quadrature AmplitudeModulation with 16 states (QAM16) conveys four information bits per RE(Q_(m)=4). A modulation order is configured to a UE by higher layersignaling from a NodeB. A modulation order is the same for all DCIformats a UE attempts to detect in a UE-DSS in a subframe or depend on aDCI format such as using Q_(m)=2 for DCI Format 0/1A and using Q_(m)=4for DCI Format 2C.

Applying a same analysis to determine a minimum ECCE size as for thefirst embodiment of the invention while considering a modulation orderof an EPDCCH transmission, a minimum ECCE size is obtained asS_(min)=O_(DCI)/(Q_(m)·R_(max)). Considering for simplicity only asingle ECCE size threshold T_(RE) and that a smallest ECCE size isgreater than T_(RE)/2, an ECCE aggregation level for which a UE mayattempt to detect an EPDCCH for a respective DCI format in a subframemay depend on a modulation order of an EPDCCH transmission in additionto T_(RE). Therefore, if an ECCE size in a respective subframe is lessthan T_(RE), a UE may not attempt detection of a DCI format for anaggregation level of one ECCE if a respective EPDCCH is transmittedusing QPSK modulation while it may attempt such detection if arespective EPDCCH is transmitted using QAM16 modulation.

FIG. 9 illustrates a dependence of an ECCE aggregation level on anEPDCCH modulation scheme, according to an embodiment of the presentinvention.

Referring to FIG. 9, either QPSK or QAM16 is assumed as a modulationscheme for an EPDCCH transmission. If an ECCE size in a respectivesubframe is less than T_(RE) in step 910, and an EPDCCH transmissionuses QPSK modulation in step 920, respective ECCE aggregation levels areL_(E) ε{2,4,8,16} ECCEs in step 930. If an EPDCCH transmission usesQAM16 modulation in step 920, respective ECCE aggregation levels areL_(E) ε{1,2,4,8} ECCEs in step 950. If an ECCE size in a respectivesubframe is greater than or equal to T_(RE) in step 910, and an EPDCCHtransmission uses QPSK modulation in step 940, respective ECCEaggregation levels are L_(E) ε{1,2,4,8} ECCEs in step 950 and step 960.If an EPDCCH transmission uses QAM16 modulation, respective ECCEaggregation levels are L_(E) ε{1,2,4} ECCEs in step 970.

A use of QAM16 modulation applies only for a transmission of a DCIformat in a UE-DSS while QPSK modulation applies for a transmission ofthe same DCI format in a CSS. For example, when a DCI Format 0/1A istransmitted through an EPDCCH in a UE-DSS, QAM16 modulation isconfigured to a UE for an EPDCCH transmission. Conversely, when a DCIFormat 0/1A is transmitted through an EPDCCH in a CSS, QPSK modulationis always used. Relying on QPSK modulation to transmit a DCI format in aCSS and configuring QAM16 modulation only for transmitting a DCI formatin a UE-DSS allows, for example, fallback support for UE scheduling whena channel experienced by a UE deteriorates enough for support of QAM16to be unreliable in practice. Additionally, QPSK modulation for anEPDCCH transmission in a CSS can be used to support UE scheduling duringan uncertainty period over which a NodeB cannot know whether a UEapplied a latest configuration of a modulation scheme (when switchingbetween QPSK and QAM16) associated with an EPDCCH transmission.

The third embodiment of the present invention considers a dependence ofECCE aggregation levels according to a presence or absence ofconfigurable information fields, variations of a resource allocationinformation field or of other information fields, or an operatingbandwidth.

A DCI format may contain information fields that are configurable byhigher layer signaling from a NodeB and, as a consequence, a DCI formatsize may vary depending on the presence or absence of such informationfields. For example, a DCI format is configured based on a presence orabsence of a field consisting of one or two bits and triggering atransmission of a reference signal by a UE for channel soundingpurposes, or of a field consisting of three bits and indicating acarrier for which a respective DCI format is intended in a multi-carriercommunication system.

An additional source of variations for a DCI format size is a maximum BWthat needs to be addressed by a respective DCI format for scheduling aPDSCH transmission to a UE or a PUSCH transmission from a UE. The sourceof this variation is a size of a Resource Allocation (RA) field in DCIformats. This RA field indicates a number of PRB pairs (or anotherresource unit such as a number of PRB pair groups), and depends on arespective total number of RBs in an operating BW. For example, for anoperating BW of 5 MHz, a RA field in a DCI Format 2C may include 13 bitswhile for an operating BW of 20 MHz, a RA field in a DCI Format 2C mayinclude 25 bits. Thus, for DCI Format 2C, although a code rate below avalue required for reliable detection can be achieved for a respectivePDCCH transmission over one ECCE when a respective PDSCH is scheduled ina small operating BW, such a code rate cannot be achieved withtransmission over one ECCE when a respective PDSCH is scheduled in alarge operating BW. However, for DCI Format 1A a code rate below a valuerequired for reliable detection can be achieved for a respective PDCCHtransmission over one ECCE, regardless of the operating BW.

At least an operating BW is considered to accommodate variations in aDCI format size when determining a threshold for an ECCE size fordefining ECCE aggregation levels for an EPDCCH transmission in asubframe. Different thresholds for an ECCE size for different BWs mayalso be defined, either by fixed values in an operation of acommunication system or by configuration to each UE through higher layersignaling from a NodeB.

FIG. 10 illustrates a dependence of an ECCE aggregation level for whicha UE attempts to detect an EPDCCH conveying a DCI format on an operatingbandwidth, according to an embodiment of the present invention.

Referring to FIG. 10, when a PDSCH transmission to a UE or a PUSCHtransmission from a UE is scheduled by a respective DCI format within arespective DL BW or UL BW that includes a first total number of PRBpairs in step 1010, a first threshold T_(RE,1) is used for an ECCE sizein order to determine ECCE aggregation levels in a respective subframein step 1020. Therefore, a first set of ECCE aggregation levels is used.Conversely, when a PDSCH transmission to a UE or a PUSCH transmissionfrom a UE is scheduled by a respective DCI format within a respective DLBW or UL BW that includes a second total number of PRB pairs in step1030, a second threshold T_(RE,2) is used for an ECCE size in order todetermine ECCE aggregation levels in a respective subframe in step 1040.Therefore, a second set of ECCE aggregation levels is used. For example,if for the first maximum number of RBs in an operating BW an ECCE sizeis less than T_(RE,1) so that a resulting code rate cannot supportreliable EPDCCH detection for a DCI format, ECCE aggregation levels fora respective EPDCCH transmission conveying the DCI format in arespective subframe is L_(E) E {2,4,8,16} ECCEs. Otherwise, ECCEaggregation levels for a respective EPDCCH transmission conveying theDCI format in a respective subframe is L_(E) ε{1,2,4,8} ECCEs.

The fourth embodiment of the present invention considers an assignmentof a different number of EPDCCH candidates for different DCI formats fora same set of ECCE aggregation levels, according to a respective size ofone or more DCI formats a UE is configured to decode by a NodeB.

A different number of EPDCCH candidates is assigned for different DCIformats for a same set of ECCE aggregation levels since a code rate fora respective EPDCCH transmission depends on a respective DCI formatpayload. For example, for an operating BW of 50 RBs, for QPSK modulation(Q_(m)=2) of an EPDCCH and for an aggregation level of two ECCEs with anECCE size of S=30 REs, a code rate for conveying a DCI Format 0/1A withan information payload of O_(DCI)=43 bits in a respective EPDCCH isR=O_(DCI)/(S·Q_(m))=0.716 (can provide reliable EPDCCH detection), whilea code rate for conveying a DCI Format 2C with an information payload ofO_(DCI)=58 bits in a respective EPDCCH using is R=0.966 (cannot providereliable EPDCCH detection). Therefore, the likelihood for an EPDCCHtransmission with an aggregation level of 2 ECCEs for the same UE andDCI format detection reliability, varies depending on whether DCI Format0/1A or DCI Format 2C is conveyed due to a variation in a respectivecode rate. Depending on their construction method, the ECCEs in a set ofPRB pairs can vary in size (typically only by a few REs). When they varyin size, the smallest ECCE is considered in the determination of thecode rate, in order to maintain a simple and robust operation.

To improve utilization for a fixed number of EPDCCH candidates a UEattempts to decode in a subframe for a respective number of ECCEaggregation levels, a distribution for such candidates to respectiveECCE aggregation levels should be adjusted for each respective DCIformat in order to address a respective difference in a resulting coderate. For example, to enable reliable detection of DCI Format 2Cconveyed by an EPDCCH, a minimum of 2 ECCEs is always required forscheduling in a large DL BW, while one ECCE is sufficient whenscheduling in a small DL BW. For DCI Format 1A, having a smaller sizethan DCI Format 2C, a code rate for a respective EPDCCH transmissionover one ECCE is sufficient in all operating DL BWs. A number of EPDCCHcandidates per ECCE aggregation level for a DCI format is either definedin an operation of a communication system or configured to a UE byhigher layer signaling from a NodeB.

FIG. 11 illustrates a dependence of EPDCCH candidates per ECCEaggregation level on a DCI format conveyed by an EPDCCH a UE attempts todetect, according to an embodiment of the present invention.

Referring to FIG. 11, when a UE decodes EPDCCHs for detecting a firstDCI format in step 1110, it considers a first set of EPDCCH candidatesfor each respective ECCE aggregation level defined in a respectivesubframe in step 1120. When a UE decodes EPDCCHs for detecting a secondDCI format in step 1130, it considers a second set of EPDCCH candidatesfor each respective ECCE aggregation level defined in a respectivesubframe in step 1140. For example, for aggregation levels L_(E)ε{1,2,4,8} ECCEs in a subframe, a number of respective EPDCCH candidatesis {6,6,2,2} for DCI Format 0/1A and is {0,8,6,2} for DCI Format 2C.

A NodeB transmitter implementing the embodiments of the presentinvention follows the conventional structure in FIG. 2, with anadditional functionality of adjusting a channel coding according to ECCEaggregation levels used in a subframe.

FIG. 12 illustrates an encoding and transmission process for a DCIformat at a NodeB transmitter incorporating an adaptation of ECCEaggregation levels per subframe, according to an embodiment of thepresent invention.

Referring to FIG. 12, based on an input of a total payload for DCIinformation and CRC bits 1210 (as described in FIG. 2) and on an inputof ECCE aggregation levels for an EPDCCH transmission in a subframe1220, a controller 1230 determines coding rates and rate matchingparameters to apply for an EPDCCH transmission. For example, if a set ofECCE aggregation levels L_(E) ε{1,2,4,8} ECCEs is used in a subframe, aconvolutional encoder applies a code rate of 2/3 for an aggregationlevel of one ECCE, a code rate of 1/3 for an aggregation level of twoECCEs, a code rate of 1/3 and one repetition for an aggregation level offour ECCEs, or a code rate of 1/3 and two repetitions for an aggregationlevel of eight ECCEs. Conversely, if a set of ECCE aggregation levelsL_(E) ε{2,4,8,16} ECCEs is used in a subframe, a convolutional encoderapplies a code rate of 1/3 without any repetition for an aggregationlevel of two ECCEs, with one repetition for an aggregation level of fourECCEs, two repetitions for an aggregation level of eight ECCEs, andeight repetitions for an aggregation level of sixteen ECCEs.Subsequently, a channel encoder 1240 applies a selected code rate with aselected number of repetitions, a rate matcher 1250 maps encoded bits ofa DCI format to allocated resources, the bits are interleaved andmodulated by an interleaver and a modulator 1260, and a control signal1270 is transmitted.

A UE receiver implementing the embodiments of the present inventionfollows the conventional structure in FIG. 3 with an additionalfunctionality of adjusting a channel decoding according to ECCEaggregation levels used in a subframe.

FIG. 13 illustrates a reception and decoding process for a DCI format ata UE receiver incorporating an adaptation of ECCE aggregation levels persubframe, according to an embodiment of the present invention.

Referring to FIG. 13, a UE receives a control signal 1220 and, based onan input of ECCE aggregation levels for an EPDCCH transmission in asubframe 1320 (as determined by a respective ECCE size and a payload ofa respective DCI format, or a modulation scheme, or a scheduling BW), acontroller 1330 determines respective demodulation and de-interleavingparameters 1340, rate de-matching parameters 1350, and channel decodingparameters 1360. Decoded DCI information and CRC bits 1370 are thenconsidered for further processing as described in FIG. 3. For example,if a set of ECCE aggregation levels L_(E) ε{1,2,4,8} ECCEs is used in asubframe, a convolutional decoder may use a code rate of 2/3 for anaggregation level of one ECCE, a code rate of 1/3 for an aggregationlevel of two ECCEs, a code rate of 1/3 with one repetition for anaggregation level of four ECCEs, or a code rate of 1/3 and tworepetitions for an aggregation level of eight ECCEs. Conversely, if aset of ECCE aggregation levels L_(E)ε{2,4,8,16} ECCEs is used in asubframe, a convolutional decoder may use a code rate of 1/3 without anyrepetition for an aggregation level of two ECCEs, with one repetitionfor an aggregation level of four ECCEs, with two repetitions for anaggregation level of eight ECCEs, and with eight repetitions for anaggregation level of sixteen ECCEs.

While the present invention has been shown and described with referenceto certain embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for a user equipment (UE) to receivecontrol information, comprising: identifying a downlink controlinformation (DCI) format to be monitored; identifying a number ofenhanced physical downlink control channel (EPDCCH) candidates based onthe identified DCI format; and receiving control information based onthe identified number of EPDCCH candidates.
 2. The method of claim 1,wherein the identifying of the aggregation level is further based on anumber of physical resource block (PRB) pairs.
 3. The method of claim 2,wherein the number of PRB pairs is configured to the UE.
 4. The methodof claim 1, wherein the identified number of EPDCCH candidates comprisesa UE-specific search space.
 5. The method of claim 1, wherein a subframeor a set of suframes for an EPDCCH is configured to the UE.
 6. A methodfor a base station to transmit control information, comprising:transmitting control information, wherein the control information isreceived by a user equipment (UE) based on a number of enhanced physicaldownlink control channel (EPDCCH) candidates, and wherein the number ofEPDCCH candidates is identified by the UE based on a downlink controlinformation (DCI) format to be monitored.
 7. The method of claim 6,wherein the aggregation level is further identified based on a number ofphysical resource block (PRB) pairs.
 8. The method of claim 7, whereinthe number of PRB pairs is configured to the UE.
 9. The method of claim6, wherein the number of EPDCCH candidates comprises a UE-specificsearch space.
 10. The method of claim 6, wherein a subframe or a set ofsuframes for an EPDCCH is configured to the UE.
 11. A user equipment(UE) to receive control information, comprising: a controller configuredto identify a downlink control information (DCI) format to be monitored,and identify a number of enhanced physical downlink control channel(EPDCCH) candidates based on the identified DCI format; and atransceiver configured to receive control information based on theidentified number of EPDCCH candidates.
 12. The UE of claim 11, whereinthe controller is further configured to identify the aggregation levelbased on a number of physical resource block (PRB) pairs.
 13. The UE ofclaim 12, wherein the number of PRB pairs is configured to the UE. 14.The UE of claim 11, wherein the identified number of EPDCCH candidatescomprises a UE-specific search space.
 15. The UE of claim 11, wherein asubframe or a set of suframes for an EPDCCH is configured to the UE. 16.A base station to transmit control information, comprising: atransceiver configured to transmit control information, wherein thecontrol information is received by a user equipment (UE) based on anumber of enhanced physical downlink control channel (EPDCCH)candidates, and wherein the number of EPDCCH candidates is identified bythe UE based on a downlink control information (DCI) format to bemonitored.
 17. The base station of claim 16, wherein the aggregationlevel is further identified based on a number of physical resource block(PRB) pairs.
 18. The base station of claim 17, wherein the number of PRBpairs is configured to the UE.
 19. The base station of claim 16, whereinthe number of EPDCCH candidates comprises a UE-specific search space.20. The base station of claim 16, wherein a subframe or a set ofsuframes for an EPDCCH is configured to the UE.